Field
The present disclosure relates to optical communication equipment and, more specifically but not exclusively, to symbol timing and clock recovery for variable-bandwidth optical signals.
Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Clock recovery is performed at an optical receiver to maintain synchronization between the digital signal processing performed at the receiver and the internal clock of the received signal. The Gardner method of clock recovery is widely used in optical receivers due to its simplicity and independence of the carrier phase. In a representative implementation, the Gardner method relies on the presence of a clock tone in the timing-error signal generated to determine the sign and magnitude of the clock adjustment that is needed to maintain the synchronization. A clock tone is typically located at a frequency that is offset by 1/T from the carrier frequency, where T is the symbol period. The timing-error signal corresponding to an optical communications signal may have two such clock tones, one on each side of the carrier. However, for optical signals having a bandwidth that is close to or narrower than 1/T, the clock tones in the timing-error signal tend to be greatly attenuated, which disadvantageously degrades the performance of the Gardner method.